Conventional ultrasound imaging systems comprise an array of ultrasonic transducers which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. For ultrasound imaging, a one-dimensional array typically has a multiplicity of transducers arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beam forming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer array is employed to receive the reflected sound (receiver mode). The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate time delay (and/or phase shift) and gain to the signal from each receiving transducer.
Referring to FIG. 1, a conventional ultrasound imaging system includes a transducer array 10 comprised of a plurality of separately driven transducer elements 12, each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter (not shown). The ultrasonic energy reflected back to transducer array 10 from the object under study is converted to an electrical signal by each receiving transducer element 12 and applied separately to a beamformer 14.
The echo signals produced by each burst of ultrasonic energy reflect from objects located at successive ranges along the ultrasonic beam. The echo signals are sensed separately by each transducer element 12 and the magnitude of the echo signal at a particular point in time represents the amount of reflection occurring at a specific range. Due to the differences in the propagation paths between an ultrasound-scattering sample volume and each transducer element 12, however, these echo signals will not be detected simultaneously and their amplitudes will not be equal. Beamformer 14 amplifies the separate echo signals, imparts the proper time delay to each, and sums them to provide a single echo signal which accurately indicates the total ultrasonic energy reflected from the sample volume. Each beamformer channel 16 receives the analog echo signal from a respective transducer element 12.
To simultaneously sum the electrical signals produced by the echoes impinging on each transducer element 12, time delays are introduced into each separate beamformer channel 16 by a beamformer controller 22. The beam time delays for reception are the same delays as the transmission delays. However, the time delay of each beamformer channel is continuously changing during reception of the echo to provide dynamic focusing of the received beam at the range from which the echo signal emanates. The beamformer channels also have circuitry (not shown) for apodizing and filtering the received pulses.
The signals entering the summer 18 are delayed so that when they are summed with delayed signals from each of the other beamformer channels 16, the summed signals indicate the magnitude and phase of the echo signal reflected from a sample volume located along the steered beam. A signal processor or detector 20 converts the received signal to display data. In the B-mode (grey-scale), this would be the envelope of the signal with some additional processing such as edge enhancement and logarithmic compression. The scan converter 6 receives the display data from detector 20 and converts the data into the desired image for display. In particular, the scan converter 6 converts the acoustic image data from polar coordinate (R-.theta.) sector format or Cartesian coordinate linear array to appropriately scaled Cartesian coordinate display pixel data at the video rate. This scan-converted acoustic data is then output for display on display monitor 8, which images the time-varying amplitude of the envelope of the signal as a grey scale.
In most conventional transducer arrays the elements are arranged in a single row, spaced at a fine pitch (one-half to one acoustic wavelength on center). In the elevation direction (perpendicular to the array axis and imaging plane), single-row transducer elements are large (tens of wavelengths) and beam formation is provided by a fixed-focus acoustic lens. Conventional one-dimensional phased-array probes have excellent lateral and axial resolution, but their elevation performance is determined by a fixed aperture focused at a fixed range. The focal length of the lens is chosen to give maximum contrast resolution near the imaging range of greatest importance for the intended application of the probe. The elevation aperture is a tradeoff between contrast resolution and sensitivity near the lens focus (improved by a large aperture) and depth of field or contrast away from the focus (improved by a smaller aperture) The elevation aperture is typically 1/6 to 1/3 of the lens focus distance (.function./6 to .function./3), which gives good slice thickness (i.e., beamwidth in the plane perpendicular to the imaging plane, also referred to as "elevation beamwidth") and contrast resolution at the focus and a moderate depth of field. However, the near-field and far-field performance (elevation slice thickness and contrast resolution) of such a probe is significantly worse than the performance at the lens focus.
Various types of multi-row transducer arrays, including so-called "1.25D", "1.5D", and "2D" arrays, have been developed to improve upon the limited elevation performance of present single-row ("1D") arrays. As used herein, these terms have the following meanings: 1D) elevation aperture is fixed and focus is at a fixed range; 1.25D) elevation aperture is variable, but focusing remains static; 1.5D) elevation aperture, shading, and focusing are dynamically variable, but symmetric about the centerline of the array; and 2D) elevation geometry and performance are comparable to azimuth, with full electronic apodization, focusing, and steering. The elevation aperture of a 1.25D probe increases with range, but the elevation focusing of that aperture is static and determined principally by a mechanical lens with a fixed focus (or foci). 1.25D probes can provide substantially better near- and far-field slice thickness performance than 1D probes, and require no additional system beamformer channels. 1.5D probes use additional beamformer channels to provide dynamic focusing and apodization in elevation. 1.5D probes can provide detail resolution comparable to, and contrast resolution substantially better than, 1.25D probes, particularly in the mid- and far-field.
FIGS. 2A and 2B show a conventional 1D array 10A, with a single row of tall, narrow transducer elements 12. The ultrasound pulses are transmitted through a semi-cylindrical focusing lens 24. Each transducer element is connected to a separate beamforming channel 16 (see FIG. 1) by a respective electrical conductor 26.
FIGS. 3A and 3B show a conventional 1.5D array 10B with Fresnel row pitch and five rows 12a-12e of transducer elements. The ultrasound pulses are again transmitted through a single-focus lens 24. For a 1.5D array with a single-focus lens 24, the Fresnel row pitch minimizes the worst-case phase error (focusing error) across the aperture. If the centerline of the array is defined to be y=0 and the outer edge to be y=y.sub.max, then the row edges are at distances ((1/3).sup.1/2, (2/3).sup.1/2, 1)y.sub.max from the centerline. The signal leads 26 from the central row transducer elements are brought out for connection to a first set of beamformer channels 2a. The array elements in rows other than the central row are electrically connected in pairs, symmetric across the centerline. Signals leads 28 from each pair of intermediate row transducer elements are brought out for connection to a second set of beamformer channels 2b. Similarly, signals leads 30 from each pair of outermost row transducer elements are brought out for connection to a third set of beamformer channels 2c. The beamformer channels 2a-2c provide independent time delays, apodization and filtering for each transducer element or pair of elements in 1.5D array 10B. The outputs of the beamformer channels are combined in summer 40, analogous to summer 18 of the 1D beamformer shown in FIG. 1.
FIGS. 4A and 4B show a conventional 1.25D array 10C with five rows 12a-12e of equal-area transducer elements. In this case, the ultrasound pulses are transmitted through a multi-focus lens 32. The row edges are at distances (1/3, 2/3, 1)y.sub.max from the array centerline. For each elevational column, the paired elements from the outer rows have a summed area which is the same as the area of each element of the central row. Thus, the pairs of elements in the outer rows have the same electrical impedance and acoustic sensitivity as that of the central row elements. The multi-focus lens improves the uniformity of the elevation beam profile by focusing the central row in the near field, where only the central row is active, and the outer rows in the far field, which is the only region where they are active.
In the 1.25D array shown in FIG. 4A, a multiplicity of multiplexers 34 are respectively connected to a corresponding multiplicity of signal leads 35 (only one multiplexer and one signal lead are seen in FIG. 4A). Each signal lead 35 is connected to a respective beamformer channel (not shown in FIG. 4A). Each multiplexer 34 has three internal switches which multiplex signal leads 26, 28, and 30 to connect with signal lead 35. Each column of transducer elements is connected to a respective set of such signal leads: the central row element 12a being connected to signal lead 26; the paired elements 12b, 12c of the intermediate rows being connected in parallel to signal lead 28; and the paired elements 12d, 12e of the outermost rows being connected in parallel to signal lead 30. In practice, the pairing of elements (i.e., connection of 12b to 12c and of 12d to 12e) is accomplished within the probe head, whereas the multiplexers may be located within the probe head, at the console end of the probe cable or within the system console itself.
Because changing the state of the multiplexer switches generates noise, use of this probe typically requires three transmit-receive cycles per beam. With the multiplexer switches 34a for the center row of elements 12a closed and switches 34b and 34c open, the transmit delays are set to provide azimuthal focusing in the near field, and the near portion of the beam data is acquired. Next, switches 34a and 34b are closed, the transmit and receive delays are reconfigured, and the mid-field data is acquired using rows 12a, 12b and 12c. Finally, all the multiplexer switches are closed, the transmit and receive delays are reconfigured, and the far-field data is acquired using rows 12a-12e. Data from the three zones are spliced together in the imaging system, with care being taken to compensate for the change in sensitivity at the transition.
All multi-row 1.25D and 1.5D arrays (and annular arrays) known at present either have a large center and small outer rows, to reduce the phase error across the aperture in the far field, or have equal-area elements, so that all beamformer channels see the same electrical and acoustic impedance and all transducer elements have the same transmit and receive efficiency. The elevation height of the central rows of these transducers sets a lower bound on the near-field slice thickness and prevents these probes from achieving optimum contrast resolution in the very near field.
Thus there is a need for a transducer which provides excellent elevation performance (minimum slice thickness) throughout the imaging field. Preferably, such a transducer should be compatible with existing ultrasound imaging systems having 128 or fewer beamformer channels.